[syntax] Change "italic" to "italic, sans-serif"

We changed the grammar non-terminal font to sans-serif,
so we should update the description.

Author: tkoeppe

source/intro.tex

@@ -954,7 +954,7 @@
 \pnum
 \indextext{notation!syntax|(}%
 In the syntax notation used in this document, syntactic
-categories are indicated by \fakegrammarterm{italic} type, and literal words
+categories are indicated by \fakegrammarterm{italic, sans-serif} type, and literal words
 and characters in \tcode{constant} \tcode{width} type. Alternatives are
 listed on separate lines except in a few cases where a long set of
 alternatives is marked by the phrase ``one of''. If the text of an alternative is too long to fit on a line, the text is continued on subsequent lines indented from the first one.